A new route is now being spotlighted for producing, in one step, a methacrylic acid ester or an acrylic acid ester by reacting methacrolein or acrolein with an alcohol and molecular oxygen. This reaction is effected by reacting methacrolein or acrolein with molecular oxygen in an alcohol, in the presence of a catalyst comprising Pd.
This reaction produces water in a similar manner as the conventional esterification reaction using a carboxylic acid and an alcohol. Some of the water produced competes with the alcohol to react with an aldehyde, to produce a carboxylic acid as a by-product, and consequently, the selectivity of a carboxylic acid ester is lowered. Also, products such as water, carboxylic acids and the like are considered to be easily adsorbed on the active site of the catalyst, thereby increasingly reducing the reaction rate as the concentrations of water and carboxylic acid increases. Therefore, attempting to increase the productivity by increasing the aldehyde concentration without changing the catalyst amount, results in decreasing the reaction rate.
An approach to solving the above-mentioned problems associated with high productivity in the presence of high concentrations of an aldehyde has been to replace the reactor with a multi-stage reactor. More-over, a method in which the reaction is effected while water is removed from the reaction system has been proposed in JP-B-4-78,626. In this reference, it is disclosed that the reaction is effected while molecular sieves, which are general water-adsorbents, and the like are added to the reaction system as a means for removing water. According to this method, conversion is not reduced even at a high aldehyde concentration, and a high selectivity of methyl methacrylate or methyl acrylate is shown.
However, the present inventors have found that in order to continuously carry out the reaction by this method, recycling of the molecular sieves is indispensable, that is, the used molecular sieves are taken out of the reaction system, regenerated, and introduced again into the reaction system. The operation of recycling the absorbent on a commercial scale results in a reduction of the running operability, and when the adsorbed water is removed in the course of regenerating the absorbent, some of the methacrylic acid ester or acrylic acid ester produced by the reaction is removed together with the water, resulting in a reduction in yield. Accordingly, this method in which an absorbent, such as a molecular sieve or the like, is used is effective when the method is carried out on a small scale for a short period of time. However, when the method is carried out on a commercial scale, not only is there the need for absorbent-regenerating process equipment, but also there is product lost during the regenerating process. Furthermore, the above-mentioned method is ideally suited for a batch operation, and hence, is problematic when a continuous reaction is carried out for a long period of time.
On the other hand, the present inventors have examined a method using reaction-distillation in which azeotropic distillation with water is effected; however, the complete separation of the starting aldehyde and alcohol from the objective carboxylic acid ester has been difficult, and it has been difficult to selectively separate only water by distillation. That is to say, by the conventional technique, it has been difficult to continue the reaction while continuously removing water from the reaction system on a commercial scale.